1. Field of the Invention
The present invention relates to a plasma display panel and, more particularly, to a method and apparatus for driving a plasma display panel for widening a driving margin and improving contrast.
2. Description of the Background Art
A plasma display panel (referred to as PDP hereinafter) displays images in such a manner that ultraviolet rays generated when an inert mixed gas such as He+Xe, Ne+Xe, He+Xe+Ne or the like is discharged excite phosphors. The size of the PDP can be easily increased and its thickness can be easily reduced. Furthermore, picture quality of the PDP is improved owing to recent technical development.
Referring to FIG. 1, a conventional three-electrode AC-type surface discharge PDP includes scan electrodes Y1 to Yn, sustain electrodes Z, and address electrodes X1 to Xm intersecting the scan electrodes Y1 to Yn and sustain electrodes Z at right angles. A cell 1 displaying one of red, green and blue is formed at each of the intersections of the scan electrodes Y1 to Yn, sustain electrodes Z and address electrodes X1 to Xm. The scan electrodes Y1 to Yn and sustain electrodes Z are formed on an upper substrate (not shown). The upper substrate includes a dielectric layer and a MgO protecting layer (which are not shown) formed thereon. The address electrodes X1 to Xm are formed on a lower substrate (not shown). The lower substrate includes ribs formed thereon. The ribs prevent optical and electrical interference between horizontally adjacent cells. A phosphor layer is formed on the lower substrate and ribs. Phosphors are excited by ultraviolet rays to emit visible light. A mixed gas such as He+Xe, Ne+Xe, He+Ne+Xe or the like, required for discharge, is injected into a discharge space between the upper and lower substrates.
To realize gray scales of images, the PDP is time-division-driven such that one frame is split into sub-fields having different numbers of times of emission. Each sub-field is divided into a reset period for initializing the entire screen, an address period for selecting a scan line and selecting cells from the selected scan line, and a sustain period for producing gray scales in response to the number of times of discharge. To display an image in 256 gray scales, for example, one frame (16.67 ms) corresponding to 1/60 seconds is divided into eight sub-fields SF1 to SF8, as shown in FIG. 2. Each of the eight sub-fields SF1 to SF8 is split into the reset period, address period and sustain period, as described above. While the reset periods and address periods of the eight sub-fields are equal, the sustain period and the number of sustain pulses allocated thereto are increased at the rate of 2n (n=0,1,2,3,4,5,6,7) in the sub-fields.
FIG. 3 shows an example of waveforms of driving signals for driving the PFP. Referring to FIG. 3, a conventional PDP driving method generates a set-up discharge using a ramp-up wave RAMP-up and generates a set-down discharge using a ramp-down wave Ramp-dn in each of sub-fields SFn and SFn+1 to initialize cells.
All scan electrodes Y are simultaneously provided with the ramp-up wave Ramp-up in the reset period of each of the sub-fields SFn and SFn+1. At the same time, the sustain electrodes Z and address electrodes X are provided with 0V. The ramp-up wave Ramp-up generates the set-up discharge, which barely generates light between adjacent scan electrode Y and address electrode X and between adjacent scan electrode Y and sustain electrode Z in the cells of the entire screen. Due to this set-up discharge, positive wall charges are accumulated on the address electrodes X and sustain electrodes Z and negative wall charges are accumulated on the scan electrodes Y.
The ramp-down wave Ramp-dn following the ramp-up wave Ramp-up is simultaneously provided to the scan electrodes Y. The ramp-down wave Ramp-dn starts to decrease at a sustain voltage Vs lower than a set-up voltage Vsetup of the ramp-up wave Ramp-up and reaches a specific negative voltage. At the same time, the sustain electrodes Z are provided with a first Z bias voltage Vz1 and the address electrodes X are provided with 0V. The first Z bias voltage Vz1 can be set to the sustain voltage Vs. When the ramp-down wave Ramp-dn is provided, a set-down discharge occurs between adjacent scan electrode Y and sustain electrode Z. This set-down discharge erases wall charges unnecessary for an address discharge, among the wall charges generated during the set-up discharge.
In the address period of each of the sub-fields SFn and SFn+1, a scan pulse Scp having a negative write voltage Vw is sequentially provided to the scan electrodes Y and, simultaneously, a data pulse Dp having a positive data voltage Vd, which is synchronized with the scan pulse Scp, is supplied to the address electrodes X. The scan pulse Scp swings between a positive write voltage+Vw lower than the sustain voltage Vs and the negative write voltage Vw. The voltages of the scan pulse Scp and data pulse Dp are added to a wall voltage generated during the reset period to generate an address discharge in the cells provided with the data pulse Dp. During the address period, a second Z bias voltage Vz2 lower than the first Z bias voltage Vz1 is provided to the sustain electrodes Z.
In the sustain period of each sub field SFn and SFn+1, a sustain pulse Susp at the sustain voltage Vs is alternately provided to the scan electrodes Y and sustain electrodes Z. In the cells selected by the address discharge, the wall voltage of the cells is added to the sustain voltage Vs to generate a display discharge between adjacent scan electrode Y and sustain electrode Z whenever the sustain pulse Susp is provided. The sustain period and the number of sustain pulses can be varied with a luminance weight given to the corresponding sub-field.
After the sustain discharge, an erase signal for erasing charges left in the cells can be provided to the scan electrodes Y or sustain electrodes Z.
When the set-down discharge is finished, the set-down voltage of the ramp-down wave Ramp-dn is fixed to a potential, which is higher than the negative write voltage Vw of the scan pulse Scp by ΔV. The lamp-down wave Ramp-dn reduces positive wall charges excessively accumulated on the address electrodes X according to the set-up discharge. Thus, when the set-down voltage of the lamp-down wave Ramp-dn is fixed to the potential higher than the negative write voltage Vw, more positive wall charges can be left on the address electrodes X. Consequently, the driving waveforms of FIG. 3 can reduce the voltages Vd and Vw required for the address discharge to drive the PDP at a low voltage. The voltage applied to the sustain electrodes Z during the address period is reduced to Vz2 in order to compensate the quantity of positive wall charges excessively left on the sustain electrodes Z when the set-down voltage is increased by ΔV during the set-down discharge.
FIG. 4 shows another example of waveforms of driving signals for driving the PFP. Referring to FIG. 4, the nth sub-field SFn initializes cells of the PDP according to a set-up discharge and set-down discharge while the (n+1)th sub-field SFn+1 initializes cells according to the set-down discharge without using the set-up discharge. The address period and sustain period of each of the nth and (n+1)th sub-fields SFn and SFn+1 are substantially identical to those of FIG. 3.
In the reset period of the nth sub-field SFn, a set-up discharge is generated using the ramp-up wave Ramp-up and then a set-down discharge is generated using the ramp-down wave Ramp-dn to initialize the cells. On the contrary, in the reset period of the (n+1)th sub-field, the lamp-down wave Ramp-dn connected to the last sustain pulse of the scan electrodes Y is applied to the scan electrodes Y to initialize the cells. In the (n+1)th sub-field, a set-down discharge occurs after a sustain discharge without having the set-up discharge, differently from the nth sub-field SFn. Accordingly, the initial state of the nth sub-field SFn before addressing is different from the initial state of the (n+1)th sub-field before addressing and thus a driving margin of the PDP is narrow.
In the meantime, the waveforms of the driving signals shown in FIG. 4 can reduce an increase in a black luminance level, caused by a set-up discharge, because the set-up discharge does not occur in the (n+1)th sub-field. This improves the contrast of PDP.